Electrophotographic developers are a dry-state developer in which toner components containing materials containing a colorant and a resin binder in a dry state, and a liquid developer in which toner components are dispersed in an insulating carrier liquid.
Liquid developers allow the toner particles to form into smaller particles, so that they give excellent image quality, thereby making it suitable for commercial printing applications. In addition, in the recent years, with the increasing demands for speeding up, liquid developers with lowered viscosities are also in demand. In other words, liquid developers that are stably dispersed at smaller particle sizes and lower viscosities are in demand, and conventionally the dispersion stability as mentioned above is obtained with the use of a dispersant.
Patent Document 1 (Japanese Examined Patent Publication No. Sho-63-30057, corresponding to U.S. Pat. No. 4,224,212) discloses as a dispersant for dispersing a solid in an organic liquid, a dispersant comprising an amide or a salt formed by a reaction between a poly(lower alkyleneimine) and a polyester having a free carboxyl group, wherein at least two polyester chains are bonded to each of the poly(lower alkyleneimine) chains.
Patent Document 2 (Japanese Patent Laid-Open No. 2008-46596, corresponding to U.S. Patent Application Publication No. 2008/0014526) discloses a liquid developer in which toner particles are dispersed in an insulating liquid, characterized in that the insulating liquid containing a first vegetable oil and a reaction product formed by a transesterification reaction between a second vegetable oil and a monohydric alcohol, has excellent fusing properties of the toner particles to a recording medium.
Also, Patent Document 3 (WO 2006/118201) describes a liquid developer in which pigment-inclusion colored resin particles are dispersed in an insulating hydrocarbon-based organic solvent according to a wet-milling method using a dispersant, characterized in that the dispersant is a polyester side chain-containing carbodiimide compound in which a polyester side chain is introduced into the molecule of the carbodiimide compound via a reaction with a carbodiimide group, as a liquid developer which serves to suppress worsening influences to electric resistance of the liquid developer and triboelectric properties of toner particles minimally, to provide the liquid developer with improved dispersibility of a pigment and dispersion stability of toner particles.